- What Is The Minimum Thickness Of Metal Coating?
The minimum coating thickness considered adequate for EMI shielding using aluminum is 0.5 microns. Maximum thickness varies according to polymer substrate thickness and various aspects of the actual design; however, we typically consider 3 microns as the workable upper limit.
- Can Other Metals Be Vapor Deposited On A Polymer Film Substrate?
The short answer is "Yes." However, for technical, environmental, and business reasons, WaveZero metalizes only with aluminum. From a technical perspective, it is important to have a highly conductive material. The relative conductivity of aluminum (normalized to copper) is 0.61. There are other metals that provide higher conductivity; for instance, silver is 1.08, and gold is 0.70. However, these materials are relatively expensive, and they present environmental and recycling issues. Copper has a relative conductivity of 1.00 but is prone to corrosion and requires another protective coating (like silver) to realize value as an effective electromagnetic shielding material. Chromium has a relative conductivity of 0.66 but is comparatively expensive and not easily deployed for process and environmental reasons. Much lower on the conductivity scale are nickel (0.22) and tin (0.15). These are more expensive than aluminum though not comparable to gold or silver. By contrast to these other metals, aluminum is inexpensive, easily recyclable, and non-toxic. Very occasionally, WaveZero will utilize an extremely thin vapor deposited undercoating of Nickel-Chromium to achieve a particular technical performance.
- Are There Adhesion Issues With The WaveZero Processes For Vacuum Metalization?
No. The basis of WaveZero's process patents for EMI shielding materials and our trade secrets is our ability to apply a tenacious coating to a polymer film in an economic fashion. We do use the vacuum metalization process, which requires that we take care with the specification of polymer film substrates; we take steps to avoid the inclusion of certain additives that can appear on the surface and thus interfere with the metalization process. We take normal care to ensure that polymer film materials are not contaminated on their surfaces from either the thermoform or the die cutting processes. Handling is carefully conducted and special care is taken in regard to the cleanliness of the manufacturing environment. Close attention to these key concepts assures that the final product has excellent adhesion. Continuous process control and testing assure a consistent product.
- Can Multiple Layers Of Metal Be Added To Increase Electromagnetic Shielding Value?
"Yes." The best way to increase SE is to have both sides metalized. This increases SE by 5 dB to 30 dB depending upon frequency and substrate layer thickness. Adding multiple materials in a single layer is typically unnecessary but can be done.
- How Uniform Is The Conductive Metal Layer?
Because the metalization process is applied to an already formed structure, there is a substantially uniform distribution of material across the surface of the polymer film. Importantly, the metal is applied after the polymer film is thermoformed. Thus, the metal layer is not stretched or made thinner as a result of the thermoforming process. If the metal were applied first and then thermoformed, the metal layer would be stretched and thinned in a non-uniform manner.
- Are There Any Residual Stresses In The Metal Layer Of Form/Met?
No. Unlike electroplating and electroless plating, which can create large compressive stresses, the vacuum metalization process creates virtually stress-free metal coatings. Also, the process order used by WaveZero (that is, metalization after thermoforming) provides a virtually stress-free final structure. If the metal were applied first and then thermoformed, there would be residual stresses created in the metal layer. Also, it is possible that micro-cracking of the conductive metal layer would occur as the metal layer would have been subjected to the same stretching experienced by the polymer film. Since metals are intrinsically less able than plastics to withstand large strains, the internal relief mechanisms activated in the metal could result in the formation of microscopic defects that are highly undesirable in thin film metal structures subjected to residual stresses. The long-term stability and performance of such a structural configuration (that is, thermoforming after metalization) are problematical, and that is the primary reason why WaveZero thermoforms prior to metallization: it is the safest, most reliable route to manufacturing the best-quality EMI shield.
- I've Heard That Many Metals Oxidize. Is Oxidation Of The Surface A Problem?
Generally, all metals oxidize when exposed to air (gold being a notable exception). Nickel, tin, and aluminum, for instance, form self-stabilizing thin oxide layers when exposed to atmospheric conditions. The oxide layer thicknesses for these metals are approximately .0015 microns, or 0.15% of the thickness of a 1 micron layer of metal. An oxide film of that thickness is not sufficient to be an environmental barrier; that is, the oxide layer is too thin to prevent the conduction of an electric field through its thickness. Since the oxide is self-stabilizing, the thickness of the oxide film changes very little with time (about 5% per year under atmospheric conditions). Metal oxides have varying degrees of hardness or resistance to mechanical friction. For instance, the oxide of tin is relatively soft, which leads to concerns of fretting corrosion when tin is used for mechanical attachment. As the tin oxide surface layer is abraded, a new, fresh tin layer is exposed. Over many cycles, the tin oxide layer is flaked off, potentially affecting mechanical fit, and eventually eroding the pure tin base structure. Aluminum oxide is harder than tin oxide, and fretting corrosion is not a concern when a mechanical attachment solution is used.
- Can Aluminum Form "Whiskers" Of The Type Observed With Tin In Various Circumstances?
No. Electroplated metals, notably zinc and tin, are known to create "whiskers," which are spontaneous hair-like growths that are nearly perfect, extremely stiff, strong, single crystalline structures. Whiskers can form immediately after plating or after several years of service. The so-called "tin whiskers" problem is of increasing concern for electronic product OEMs and CEMs. Tin whiskers have been observed in lengths up to 0.4 inches though smaller lengths are more common. The exact mechanism of tin whisker growth is not well understood but it is unique to the electroplating process because of the compressive stresses created. Tin whiskers present two reliability issues for equipment manufacturers and users: electrical shorting (which can cause either a transient or permanent short) and broken/dislodged debris (which can foul mechanical and optical components). Whisker growth has recently become a critically important issue because of circuit geometry reductions, lower applied voltages, and new regulations calling for the replacement of lead-based components and processes with non-lead alternatives. According to the March 2002 paper "Tin Whiskers: Attributes and Mitigation," by Jay A. Brusse of NASA Goddard Space Flight Center and Gary J. Ewell and Jocelyn P. Siplon of The Aerospace Corporation, "Many of the proposed mechanisms of mitigation, including control of the immediate underplating material, use of conformal coating, regulating the thickness of the tin coating, use of matte tin electroplating, and annealing or fusing of the tin layer, are inadequate. They likely reduce the incidence of nucleation or growth but do not provide an absolute guarantee of lack of whisker formation."
